the call setup is done on SDCCH and then released.
One call setup lasts only 3 seconds. So the 24 channels are used repeatedly until the 90 TCH are used.
In a restaurant, there are only 2 waiters for 30 tables. The 2 waiters are serving every table once in a while.
The SDCCH is doing the same : it is serving every TCH once in a while (to locate a MS on it).
I hope the analogy works..
(no, no need to tip the SDCCH at the call release)